Studies of the structure and organization of cationic lipid bilayer membranes: Calorimetric, spectroscopic, and x-ray diffraction studies of linear saturated P-O-ethyl phosphatidylcholines

Differential scanning calorimetry, x-ray diffraction, and infrared and P-31 -nuclear magnetic resonance (P-31-NMR) spectroscopy were used to examine th e thermotropic phase behavior and organization of cationic model membranes composed of the P-O-ethyl esters of a homologous series of n-saturated 1,2- diacyl phosphatidylcholines (Et-PCs). Differential scanning calorimetry stu dies indicate that on heating, these lipids exhibit single highly energetic and cooperative endothermic transitions whose temperatures and enthalpies are higher than those of the corresponding phosphatidylcholines (PCs). Upon cooling, these Et-PCs exhibit two exothermic transitions at temperatures s lightly below the single endotherm observed upon heating. These cooling exo therms have both been assigned to transitions between the liquid-crystallin e and gel phases of these lipids by x-ray diffraction. The x-ray diffractio n data also show that unlike the parent PCs, the chain-melting phase transi tion of these Et-PCs involves a direct transformation of a chain-interdigit ated gel phase to the lamellar liquid-crystalline phase for the homologous series of n greater than or equal to 14. Our P-31-NMR spectroscopic studies indicate that the rates of phosphate headgroup reorientation in both gel a nd liquid-crystalline phases of these lipids are comparable to those of the corresponding PC bilayers. However, the shape of the P-31-NMR spectra obse rved in the interdigitated gel phase indicates that phosphate headgroup reo rientation is subject to constraints that are not encountered in the non-in terdigitated gel phases of parent PCs. The infrared spectroscopic data indi cate that the Et-PCs adopt a very compact form of hydrocarbon chain packing in the interdigitated gel phase and that the polar/apolar interfacial regi ons of these bilayers are less hydrated than those of corresponding PC bila yers in both the gel and liquid-crystalline phases. Our results indicate th at esterification of PC phosphate headgroups results in many alterations of bilayer physical properties aside from the endowment of a positively charg ed surface. This fact should be considered in assessing the interactions of these compounds with naturally occurring lipids and with other biological materials.